1. Technical Field
The present invention relates to a membrane-electrode assembly, a direct carbon fuel cell including the same, and a method of preparing the same.
2. Description of the Related Art
Carbon exists in the solid state in nature so that it has an advantage in that it has higher energy density than hydrogen existing in the gas state, for example. However, it is difficult to make reaction occur between solid carbon and a solid anode catalyst at a three-phase boundary (TPB) in contact with a porous catalyst. With reference to documents relating to a direct carbon fuel cell (DCFC) which uses coal as a fuel, the operating methods of a fuel cell may vary depending on its system configuration and the kinds of its electrolytes. These methods were contemplated in an effort to overcome lower reactivity at the anode electrochemical reaction interface in a fuel cell using coal as a fuel.
Methods have been proposed that make a coal as its gaseous phase or make use of an electrolyte to contact liquidus molten carbonates with carbon, such that the contact between a solid fuel and a solid anode may be improved. However, such methods compromise thermodynamic advantages that produce at least 80% efficiency in a direct electrochemical reaction. This is because chemical energy from carbon is not directly used electrochemically, but CO from a chemical reaction, e.g., Reverse Boudouard reaction: C+CO2→CO, is used as a fuel, or steam or oxygen was introduced to facilitate an internal chemical reforming reaction when fueled for steam reforming or partial oxidation. A systematically crucial factor in DCFCs is reducing an ohmic resistance between a membrane-electrode assembly (MEA) and a current collector, and charging a solid carbon fuel therefor. This is because, at an anode, good contact between the solid carbon fuel and a catalytic reaction site of the anode allows electrochemical reaction to easily occur. To this end, however, current collecting methods used in a conventional solid oxide fuel cell (SOFC) may not be employed in the same way to a direct carbon fuel cell, because sufficient reaction sites may not be obtained in such case. Although many researches are now ongoing for a direct electrochemical oxidation of a solid carbon fuel in DCFC, but little achievement has been made so far to couple MEA with a current collector as well as to charge a fuel. Desclaux et al. discloses a direct carbon fuel cell having a structure as shown in FIGS. 1A and 1B, which includes a solid coal, a Pt mesh and a Pt wire disposed on a fuel-electrode(anode)/electrolyte/air-electrode(cathode) structure (International Journal Hydrogen Energy 36 (2011) 10278-10281). In this case, since the Pt wire is in contact only with the solid coal, but not with an adhesive, as well as not with the anode, the electron transfer resistance at the interface is high.
Chen Li et al. discloses a direct carbon fuel cell having a structure as shown in FIG. 2, which includes (Ag or Pt adhesive)/(Pt mesh+Pt wire)/(solid coal layer) disposed on a fuel-electrode(anode)/electrolyte/air-electrode(cathode) (Journal of Power Sources 196 (2011) 4588-4593). In this case, although the Pt wire is connected with the Pt mesh through a coal layer, and the Pt mesh is intimately connected with the anode by the Ag adhesive, since the coal fuel needs to be provided to the anode through the Pt mesh and Ag adhesive layer, the fuel transfer resistance at the interface is high.